Soybean Rust Diversity and Adaptation of Elite Soybean Lines to the Ugandan Environment

Soybean rust (Phakopsora pachyrhizi Sydow) is a major threat to soybean production worldwide. The objectives of this study were to; a) assess P. pachyrhizi diversity in the major soybean growing areas of Uganda usin SSR markers; b) assess elite soybean lines for resistance against geographically diverse rust bulk isolates under screen house conditions; c) determine the adaptation and stability of selected elite soybean lines in five diverse locations of Uganda. Rust isolates were collected from five locations that are the major soybean growing areas. Total DNA of the rust isolates was extracted. Twenty four elite soybean lines were inoculated with rust isolates from four locations (Iki-Iki, Mubuku, NaCRRI and Nakabango) using detached leaf technique. To determine the adaptation and stability of the 24 elite soybean lines, a study was conducted for four consecutive seasons. A dendrogram constructed from the similarity data using NTSYSpc Version 2.2 showed that the isolates from the five locations were grouped into two clusters. Three distinct reaction types (tan, reddish brown and mixed) were detected. Genotype Maksoy 3N showed reddish brown reaction type for isolates from three sites (Iki-Iki, Mubuku and Nakabango). Maksoy 3N had the lowest mean rust score of 3.1; followed by DXT 5.16 that had a mean score of 3.5. Mubuku and Iki-Iki had the highest mean rust scores of 4.7 and 3.7 respectively while NaCRRI had the lowest mean score of 2.8. Season 2011A had the lowest mean score of 2.8; followed by season 2010A (3.3), 2010B (4.1) and season 2011B had the highest (5.0). Combined yield of the 24 soybean genotypes for three seasons across the five locations indicated that Maksoy 3N had the highest mean yield of 1703 kg ha-1; followed by DXT 3.11 (1576 kg ha-1), DXT SPS 7.11-1 (1549 kg ha-1) and DXT SPS 2.15-12 (1432 kg ha-1). The scatter plot from GGE analysis indicated that all the five locations were in one mega environment with the best genotype being Maksoy 3N. Comparison biplots showed that DXT 5.16 is the most stable genotype while Maksoy 3N is the most ideal genotype and Nakabango, the most ideal environment. This study showed that the SSR markers developed by Anderson et al. (2008) can be used to study rust diversity in Uganda. The 24 elite soybean lines showed different reaction types when inoculated with bulk rust isolates from different locations in Uganda. Field observations showed that soybean rust was the major soybean disease in all soybean growing areas in Uganda. Maksoy 3N that was included as the highest yielding recommended variety in Uganda had the highest yields and was the most ideal genotype. All the test locations were grouped into one mega environment by GGE biplot. In future rust diversity studies, the number of fields sampled should be increased and the rust isolates should be collected from specific plants per location within the fields. Maksoy 3N should be used to improve other soybean varieties for soybean rust resistance and yield through hybridization.